1. Field of the Invention
The present invention relates to a structure having recesses and projections, a method of manufacturing the structure, and a functional device utilizing the structure. Also, the present invention relates to a method of manufacturing a nano-structure having a nano-scale pitch.
2. Description of the Related Art
Hitherto, manufacturing a porous structure with the anodic oxidation process has received attention as a simple method for forming a structure having recesses and projections of from nanometer to micrometer sizes. The anodic oxidation process generally comprises the steps of immersing a workpiece in an acidic solution, and applying a voltage with the workpiece serving as an anode to cause phenomena of dissolution and oxidation of the workpiece surface at the same time, thereby forming a microscopic recessed/projected structure. This microscopic structure is known as exhibiting regularity through self-assembly under certain conditions (such as voltage, temperature and kind of acid), and applications of the microscopic structure to various nano-devices, such as a recording medium and an optical functional device, are expected.
One practical example of the anodic oxidation process is anodic oxidation of aluminum. When a voltage is applied to a bath of an aqueous solution of oxalic acid, phosphoric acid, or sulfuric acid with aluminum serving as an anode, nanoholes having wall surfaces given by a barrier layer (alumina) are formed at a nanometer-size pitch and a porous coating is obtained. While a similar phenomenon may also occur with another kind of metal, it is generally known that columnar pores (nanoholes) with the best straightness can be obtained using aluminum. The anodic oxidation of aluminum can form pores in a regular array in which the pore pitch is several tens nanometers (“Appl. Phys. Lett.”, 72, 1173, 1998), or in an irregular array in which the average pore pitch is about 18 nm and the pore diameter is 8 nm (Japanese Patent Laid-Open No. 2000-195036). The pores in a regular array are obtained by carrying out the anodic oxidation while holding the conditions, such as the kind of acid, concentration, temperature and the applied voltage, at certain values, but a long time is required until the pores are formed in a regular array through self-assembly alone.
To improve the above-mentioned problem, there has been so far proposed a method of forming regularly arrayed recesses in the surface of a workpiece in advance by any suitable method, and carrying out anodic oxidation with the recesses serving as start points for pore formation by the anodic oxidation. This method can produce vertical pores in a very short time, which are arranged in a regular array and are directly extended from the workpiece surface. As shown in FIGS. 5A to 5C, Japanese Patent-Laid Open No. 10-121292 proposes a method comprising the steps of fabricating a stamper 100 having regularly arrayed projections 101 by, e.g., photolithography or electron beam drawing (see FIG. 5A), and forming recesses 103 by pressing, which serve as start points for pore formation, in a transfer member 102, i.e., a substrate which is subjected to the anodic oxidation (FIGS. 5B and 5C).